These docs are for an old release.
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weekNumberCalculation 1.6
The method for calculating week numbers that are displayed with the weekNumbers setting.
String/Function, default:"iso"
The default ("iso") causes ISO8601 week numbers.
You may also specify a function, which accepts a single native Date object and returns an integer week number. Since FullCalendar currently lacks a built-in internationlization system, this is only way to implement localized week numbers.